1. Field of the Invention
An oxide semiconductor thin film transistor (TFT) and a method of manufacturing the same where the channel layer includes an oxide semiconductor material.
2. Description of the Related Art
TFTs have been applied in various fields. In particular, TFTs are used as switching and driving devices in display devices and as selection switches in cross point type memory devices.
Liquid crystal displays (LCDs) are mainly used as television (TV) panels. Therefore, much research has been conducted in order to apply organic light emitting display (OLED) technology to TVs. TV display technology has been developed to satisfy the requirements of the display market, such as large-scale TVs or Digital Information Displays (DIDs), low prices, high definition (moving picture expression, high resolution, brightness, contrast ratio, color representation, etc.), etc. In order to cope with these requirements, large-sized substrates such as glass, etc., are required as well as high performance TFTs for use as switching and driving devices.
Amorphous silicon (a-Si) TFTs are widely used in to produce low cost driving and switching devices that can be implemented uniformly across large displays having lengths of 2 meters. However, such a-Si TFTs are required to have a high performance in view of the tendency to produce high definition characteristics in a large-sized display device. However, a-Si TFTs technology is limited in that the mobility can not exceed 0.5 cm2/Vs. Accordingly, high-performance TFTs having higher mobility than a-Si TFTs and manufacturing techniques for such TFTs are required.
Since poly silicon (p-Si) TFTs have high mobility of several tens to hundreds of cm2/Vs, the p-Si TFTs, unlike the a-Si TFTs, are applied to high definition displays. Also, the characteristics of p-Si TFTs deteriorate much less than do the characteristics a-Si TFTs. However, more complicated processes are required to manufacture p-Si TFTs than a-Si TFTs, which increases the manufacturing costs. Therefore, p-Si TFTs are appropriate for high definition displays and products such as OLEDs. However, p-Si TFTs are inferior to a-Si TFTs in terms of cost and thus have limited applications in displays. Also, in the case of p-Si TFTs, manufacturing processes using large substrates having lengths of 1 meter or more have not been introduced due to limits in manufacturing equipment and technical problems such as poor uniformity. Thus, it is difficult to apply p-Si TFTs to TV products.
Therefore, what is needed is a new TFT technology that can combine together the advantages of a-Si TFTs and p-Si TFTs so that a large screen, high definition TV display can be produced at reduced manufacturing costs.
Recently, ZnO-based TFTs have been in the spotlight as an oxide semiconductor device. Zn Oxide, Ga—In—Zn Oxide, etc., have been introduced as ZnO-based materials. Since ZnO-based semiconductor devices are manufactured using low temperature processes that have amorphous phases, ZnO-based semiconductor devices can be easily formed over large areas. Like p-Si, ZnO-based semiconductor films have superior electrical characteristics such as high mobility. As a result, research has been conducted to try to use oxide semiconductor material layers having high mobility, such as ZnO-based material layers, into channel areas of TFTs. Zn Oxide, Ga—In—Zn Oxide, etc., are introduced as ZnO-based materials.